1. Field of the Invention
This invention relates to a magnetic recording medium which comprises a nonmagnetic support having thereon a magnetic layer comprising a ferromagnetic powder dispersed in a binder, wherein the nonmagnetic support is made of an aromatic polyamide resin and, more particularly, to a magnetic recording medium hardly sustaining improper tape winding or edge damages.
2. Description of Prior Art
Magnetic recording media are required to satisfy various characteristics. For example, a magnetic recording tape used for an audio tape for reproduction of music recordings requires high original sound reproduction capability, while a magnetic recording tape used for a videotape requires high original picture reproduction capability. In addition to such electromagnetic characteristics, the magnetic recording tape requires good running performance and running durability. Recently, such a magnetic recording tape requires to record signals with high density. For media for computer storage, particularly, for compact backup system tapes as represented by the D8 and the DDS, the media are required to have a thinner total tape thickness for a large capacity.
To response various demands as above, various improvements are added to respective structural elements of the magnetic recording media. The magnetic recording medium generally has a magnetic layer on one side of the nonmagnetic support (optionally, a nonmagnetic undercoating layer may be formed when necessary) and a back-coating layer on the other side of a nonmagnetic support. To cope with downsizing and higher density recording, not only the magnetic layer but also the entire layers constituting the magnetic recording medium are required to be thinner. To make the thickness of the magnetic recording tape thinner, conventionally, the nonmagnetic support was made thinner, or the nonmagnetic layer formed between the nonmagnetic support and the magnetic layer was made thinner. If the nonmagnetic support is made thinner below a certain range, however, the medium loses the running durability. If the nonmagnetic layer is made thinner, the medium suffers from lowered outputs, increased error rates, and increased dropouts. Therefore, for the magnetic recording tapes for the DDS-2 and DDS-3 formats, aramid based supports (see, Japanese Patent No. 2,724,581) came to be used as a nonmagnetic support for improving the electromagnetic characteristics and the running durability, instead of polyester based supports such as polyethyleneterephthalate (PET) and polyethylenenaphthalate (PEN), which are used conventionally. Particularly, when the tape thickness is made thinner, it becomes difficult to keep strong contact of the head with the tape surface, and it is therefore desirable to use a support having a high module of elasticity. Consequently, aromatic polyamide (aramid) came to be used these days.
However, we have discovered that when the aromatic polyamide is used as the nonmagnetic support, tape edges of the magnetic recording tape become thicker as the tape runs more frequently, thereby creating an improper winding or thereby sustaining edge damages that the tape edges are worn out. This would cause the tape to be unable to run. This tendency is remarkable in a videotape for business use in which cueing, reviewing, and so on are frequently used.
Thus, in magnetic recording media using an aromatic polyamide as a nonmagnetic support, occurrences of improper windings and edge damages raise serious problems. Therefore, it is an object of the invention to provide a magnetic recording medium using an aromatic polyamide film as a nonmagnetic support and hardly sustaining improper windings and edge damages.
The inventors have diligently researched magnetic recording media to accomplish the above object. Consequently, the inventors have found out that the problems of sustaining improper windings and edge damages could be solved by selection of an aromatic polyamide having a loss tangent satisfying prescribed conditions for a magnetic recording medium even where an aromatic polyamide is used for nonmagnetic support, and reaches the completion of the invention.
According to the invention, a magnetic recording medium comprises a nonmagnetic support made of aromatic polyamide resin, and a magnetic layer formed on the nonmagnetic support containing ferromagnetic powders dispersed in a binder, wherein the magnetic recording medium has a loss tangent at the temperature of 40xc2x0 C. is 0.7 or higher with respect to a loss tangent at the temperature of 100xc2x0 C.
In the above magnetic recording medium, the loss tangent at the temperature of 40xc2x0 C. is preferably 1 or higher with respect to a loss tangent at the temperature of 100xc2x0 C., and the loss tangent at the temperature of 100xc2x0 C. is preferably 0.05 or lower.
In another aspect of the invention, a magnetic recording medium comprises a nonmagnetic support made of aromatic polyamide resin, and a magnetic layer formed on the nonmagnetic support containing ferromagnetic powders dispersed in a binder, wherein the magnetic recording medium has a loss tangent at the temperature of 100xc2x0 C. of 0.05 or lower. In this magnetic recording medium, the loss tangent at the temperature of 100xc2x0 C. is preferably 0.01 or higher.
In any magnetic recording medium according to the invention, a nonmagnetic layer mainly containing nonmagnetic inorganic powders and a binder may be formed on the nonmagnetic support, and the magnetic layer containing the ferromagnetic powders and the binder and having a thickness of 0.05 xcexcm or larger and 1.0 xcexcm or lower may be formed on the nonmagnetic layer.
The magnetic recording medium according to the invention comprises a nonmagnetic support and a magnetic layer formed on the nonmagnetic support containing ferromagnetic powders dispersed in a binder, and is characterized in using an aromatic polyamide resin as the nonmagnetic support and having a loss tangent at the temperature of 40xc2x0 C. of 0.7 or higher with respect to a loss tangent at the temperature of 100xc2x0 C. In this magnetic recording medium according to the invention, the above problems can be solved by selecting an aromatic polyamide resin having a loss tangent at the temperature of 40xc2x0 C. of 0.7 or higher with respect to a loss tangent at the temperature of 100xc2x0 C. [0.7xe2x89xa6tanxcex4 (40xc2x0 C.)/tanxcex4 (100xc2x0 C.)]. Preferably, 0.7xe2x89xa6tanxcex4 (40xc2x0 C.)/tanxcex4 (100xc2x0 C.)xe2x89xa62, and more preferably, 0.8xe2x89xa6tanxcex4 (40xc2x0 C.)/tanxcex4 (100xc2x0 C.)xe2x89xa61.5.
To raise the capability of editing in a videotape for business use, the tape should be run in various running styles. During cueing mode or reviewing mode, the tape is made to run at a high speed even though a short period of time, and the tape becomes a high temperature because the tape is subject to high speed contacts, and conversely, during a normal play mode, the tape receives contacts for a long period of time, though the tape is subject to weak contacts and a relatively lower temperature. Running of the tape at a high temperature largely influences viscoelasticity of the tape but the property at a relatively low temperature is also important. To make stable the tape running at a high temperature though a short period of time and the tape running at a relatively low temperature for a long period of time, it is preferable to set the loss tangent indicating the viscoelasticity of the magnetic recording medium constant notwithstanding the temperature or frequency. The above problem can be solved by rendering the magnetic recording medium satisfy the above condition of 0.7xe2x89xa6tanxcex4 (40xc2x0 C.)/tanxcex4 (100xc2x0 C.).
Generally, a loss tangent of a magnetic recording medium is relatively low at 40xc2x0 C. or around room temperature, although the loss tangent has tendency to increase as temperature increases. Accordingly, in the magnetic recording medium of the invention, in addition to the rate of tanxcex4 (40xc2x0 C.)/tanxcex4 (100xc2x0 C.), the loss tangent (absolute value) at the temperature of 100xc2x0 C. is also preferably small at 0.05 or lower.
The magnetic recording medium of the invention comprises a nonmagnetic support made of aromatic polyamide resin, and a magnetic layer formed on the nonmagnetic support containing ferromagnetic powders dispersed in a binder, wherein the magnetic recording medium has a loss tangent at the temperature of 100xc2x0 C. of 0.05 or lower.
As described above, to gain a stable running property, a loss tangent of a proper viscoelasticity is required, and it is desirable that the loss tangent is constant notwithstanding temperature and frequency. The loss tangent generally has a tendency that the loss tangent is relatively low at 40xc2x0 C. around a room temperature but increases as the temperature goes up. In the magnetic recording medium according to the invention, the above problem can be solved by selecting the nonmagnetic support made of the aromatic polyamide resin to be made of a material whose tan xcex4 (100xc2x0 C.) is 0.05 or lower. The loss tangent of the magnetic recording medium at 100xc2x0 C. is preferably 0.01 or higher.
The aromatic polyamide film used in this invention is a film base obtained from making an aromatic polyamide into a film. The aromatic polyamide can be, for example, a material containing 50% or more of units as indicated by a general formula,
xe2x80x94NHCOxe2x80x94Ar1xe2x80x94CONHxe2x80x94Ar2
wherein, Ar1 and Ar2 are bivalent organic groups having at least one aromatic ring whose carbon number is preferably within 6 to 25; or
xe2x80x94COxe2x80x94Ar3xe2x80x94NHxe2x80x94
wherein, Ar3 is a bivalent organic group having at least one aromatic ring whose carbon number is preferably within 6 to 25. For example, paraphenyleneterephthalamide, paraphenyleneisophthalamide, metaphenyleneterephthalamide, metaphenyleneisophthalamide, and so on are exemplified. Furthermore, the material also includes materials having substituents such as a nitro group, an alkyl group, an alkoxyl group, or the like on the phenyl nucleus. Among those aromatic polyamides, a material essentially having the paraphenyleneterephthalamide is preferable, which has a strong mechanical strength, a high modulus of elasticity, a low moisture absorptibity, a good heat resistance, and a good size stability from mechanical and thermal aspects, and therefore, the material is suitable for a material for good high density recording medium.
As a monomer constituting the aromatic polyamide thus structured, an acid chloride such as terephthalic acid chloride and the like, and a diamine such as paraphenylenediamine, metaphenylenediamine and the like can be exemplified. Particularly, an aromatic polyamide film used for the nonmagnetic support in this invention is preferably one not containing chlorine and is more preferably made of a PPTA (polyparaphenyleneterephthalamide) as represented by a formula
xe2x80x94(COxe2x80x94Phxe2x80x94CONHxe2x80x94Ph)nxe2x80x94.
The above aromatic polyamide is set forth in, e.g., Japanese Patent No. 2628898. Such an aromatic polyamide is commercially available, and for example, Aramica (trademark) made by Asahi Chemical Industry Co., Ltd. can be exemplified. The thickness of the aromatic polyamide film used in this invention is generally in a range of 1.0 to 10 xcexcm, preferably, 2.0 to 6.0 xcexcm, and more preferably, 3.0 to 5.0 xcexcm. The magnetic recording medium of the invention has a proper thickness of the entire layers of 2 xcexcm or above and less than 7 xcexcm, preferably, 2 xcexcm or above and less than 6.8 xcexcm, from a viewpoint of obtaining a thinner medium and with a higher density in use of an aramide film.
The magnetic layer of the magnetic recording medium of the invention has a structure that ferromagnetic powders are dispersed in a binder. The ferromagnetic powders used here are, e.g., powders of ferromagnetic iron oxide, cobalt containing ferromagnetic iron oxide, barium ferrite, and ferromagnetic metal powders. The ferromagnetic powders have an SBET (BET specific surface area) of 40 to 80 m2/g, preferably 50 to 70 m2/g. The crystallite size is 12 to 25 nm, preferably 13 to 22 nm, more preferably 14 to 20 nm. The major axis length is of 0.05 to 0.25 xcexcm, preferably, 0.07 to 0.2 xcexcm, more preferably, 0.08 to 0.15 xcexcm. The pH of the ferromagnetic powders is preferably 7 or higher. As ferromagnetic metal powders, exemplified are a simple substance or alloy of Fe, Ni, Fexe2x80x94Co, Fexe2x80x94Ni, Coxe2x80x94Ni, Coxe2x80x94Nixe2x80x94Fe, etc. and the powders may contain, in a range in 20% or less by weight of metal component or components, aluminum, silicon, sulfur, scandium, titanium, vanadium, chrome, manganese, copper, zinc, yttrium, molybdenum, rhodium, palladium, gold, tin, antimony, boron, barium, tantalum, tungsten, rhenium, silver, lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, bismuth, and so on. The magnetic powders may contain as described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-255334 which corresponds to U.S. application Ser. No. 08/862,262 filed May 23, 1997, Co of 10 to 40 atomic %, Al of 2 to 20 atomic %, and Y of 1 to 15 atomic %, with respect to Fe, from a viewpoint of improvements of dispersing property by reducing sintered portions. The ferromagnetic powders may contain a small amount of water, hydrides, and oxides.
The ferromagnetic powders used in the magnetic layer of the magnetic recording medium of the invention preferably have Fe as an essential component, a length of major axis of 0.05 to 0.19 xcexcm, a crystallite size of 100 to 230 xc3x85 from a viewpoint of reduction of noises in filling the magnetic powders with a high density. The ferromagnetic powders used in the magnetic layer of the invented magnetic recording medium has a coercive force of 1650 to 3000 Oe, and "sgr"s of 125 to 180 emu/g from a viewpoint of reduction of recording demagnetization loss and prevention of reduction of the magnetization amount due to heat deviations. Furthermore, the SSA (specific surface area) of the ferromagnetic powders is preferably, 35 to 60 m2/g in terms of appropriate disperse liquid viscosity and affinity to the binder. Some methods for manufacturing such ferromagnetic powders are already known publicly, and the ferromagnetic powders used in this invention can be manufactured according to the known methods.
There is no special limitation to the shape of the ferromagnetic powders, but ordinarily, the powders in an acicular shape, a grain shape, a dice shape, a rice grain shape (or may be called as a spindle shape) or a plate shape may be used. Particularly, ferromagnetic powders in an acicular shape or a spindle shape are preferably used.
In this invention, the binder, the hardening agent, and the ferromagnetic powders are kneaded and dispersed together with a solvent or solvents such as methyl ethyl ketone, dioxane, cyclohexanone, ethyl acetate, toluene which are ordinarily used in preparation of magnetic paints to form paints for formation of magnetic layers. Such kneading and dispersing operation can be done in an ordinary fashion.
The binder usable in the magnetic layer of the magnetic recording medium of the invention can be conventionally known, thermoplastic resins, thermosetting resins, and reactive resins. Preferable binders are vinyl chloride resin, vinyl chloride-vinyl acetate resin, fiber based resin such as nitrocellulose, phenoxy resin, polyurethane resin, and so on. It is preferable, among those, to use vinyl chloride resin, vinyl chloride-vinyl acetate resin, and polyurethane resin since they can reduce transfers on the back side where the hardness of the back coating layer is made closer to the hardness of the magnetic layer. It is also preferable for the binder to contain as a part a polyurethane resin containing ring structures and ether groups in terms of improvements of dispersion property.
Particularly, desirable binders are a polyurethane resin obtained from reaction between a diol and an organic diisocyanate resin. The polyurethane resin contains a short chain diol having a ring structure or structures and a long chain diol having an ether linkage or linkages, as a diol, of 17 to 40% by weight and 10 to 50% by weight, respectively, with respect to the polyurethane resin, and the ether linkage in the long chain diol, of 1.0 to 5.0 mol/g with respect to the polyurethane resin. Hereinafter, the polyurethane resin is described.
The short chain diol has a molecular weight 50 or higher and less than 500, more preferably, 100 to 300. As specific examples, aromatic or alicyclic diols such as a cyclohexane-1, 4-diol, cyclohexane-1, 4-dimethanol, bisphenol A, bisphenol A hydride, bisphenol S, bisphenol P, those added with ethylene oxide, or propylene oxide, cyclohexane dimethanol, and cyclohexane diol can be exemplified.
The long chain diol has a molecular weight of 500 or higher and less than 5000. As specific examples, exemplified are bisphenol A and bisphenol A hydride added with ethylene oxide or propylene oxide, having a molecular weight of 500 or higher and less than 5000. A desirable short chain diol and long chain diol are indicated by the following Formula (1): 
In a case of short chain diols, the numerals of xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d are so selected that the molecular weight of the short chain diol is of 50 or higher and less than 500. In general, they are 0 to 3. In a case of long chain diols, the numerals of xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d are so selected that the molecular weight of the long chain diol is of 500 or higher and less than 5,000. In general, they are 3 to 24, preferably 3 to 20, more preferably 4 to 15. If the numerals of xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d become larger than 24, the back coat layer is softened, and the running durability may be lowered. xe2x80x9cXxe2x80x9d preferably represents a hydrogen atom or methyl group, and more preferably, a methyl group. xe2x80x9cXxe2x80x9d in the parentheses with xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d does not necessarily represent the same group, and hydrogen atoms and methyl groups can coexist.
The desirable short chain diol as shown in Formula (1) is bisphenol A, hydrogenated bisphenol A and adducts of those with ethylene oxide or propylene oxide. The desirable long chain diol is a diol having a molecular weight of 500 to 5000 derived from bisphenol A or hydrogenated bisphenol A, more preferably, an adduct of bisphenol A with propylene oxide.
The content of the short chain diol is of 17 to 40% by weight with respect to the polyurethane resin, more preferably, 20 to 30% by weight. The content of the long chain diol is of 10 to 50% by weight, more preferably, 30 to 40 by weight.
The ether group of the long chain diol exists in an amount of 1.0 to 5.0 mmol/g in the polyurethane resin, more preferably 2.0 to 4.0 mmol/g. This makes adhesive property to the particles better and dispersing property better. In addition, this makes solubility to the solvent better.
Diols other than short chain diols and long chain diols can be used together. More specifically, aliphatic diols such as ethylene glycol, 1,3-propylenediol, 1,2-propyleneglycol, 1,4-butandiol, 1,5-pentandiol, 1,6-hexanediol, 2,2-dimethylpropanediol, 1,8-octanediol, 1,9-nonandiol, and diethyleneglycol, and an adduct of N-diethanolamine with ethylene oxide or propylene oxide can be exemplified.
As examples of organic diisocyanate compound to be reacted, exemplified are aromatic diisocyanates such as 2,4-trilenediisocyanate, 2,6-trilenediisocyanate, xylene-1,4-diisocyanate, xylene-1,3-diisocyanate, 4,4xe2x80x2-diphenylmethanediisocyanate, 4,4-diphenyletherdiisocyanate, 2-nitrodiphenyl-4,4xe2x80x2-diisocyanate, 2,2xe2x80x2-diphenylpropane-4,4xe2x80x2-diisocyanate, 4,4-diphenylpropanediisocyanate, m-phenylenediisocyanate, p-phenylenediisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3xe2x80x2-dimethoxydiphenyl-4,4xe2x80x2-diisocyanate, aliphatic diisocyanates such as lysinediisocyanate, and alicyclic diisocyanates such as isophoronediisocyanate, hydrogenated trilenediisocyanate, and hydrogenated diphenyletherdiisocyanate.
Since the polyurethane resin obtained through reactions has a ring structure portion or portions, the magnetic layer prepared in use of this resin has a higher strength, a higher glass transition temperature, and a higher durability. Where branched methyl is introduced, the dispersing property will be improved since the solubility to a solvent is improved.
The glass transition temperature Tg of the polyurethane resin is of xe2x88x9220 to 150xc2x0 C., preferably 20 to 120xc2x0 C., more preferably 50 to 100xc2x0 C. It is preferable to prepare the binder composition so that both of proper calendar formation and proper coating strength are accomplished by rendering the glass transition temperature Tg of the coating film an optimum value in which the glass transition temperature Tg is of 50 to 150xc2x0 C., preferably 70 to 100xc2x0 C. and in which the calendar processing temperature +30xc2x0 C. is equal to the glass transition temperature Tg of the coating film, where the long chain diol has a ring portion made of either an aliphatic or aromatic compound.
The binder is generally hardened by a polyisocyanate hardener. The used amount of the hardener, with respect to 00 parts by weight of the polyurethane resin is 0 to 150 arts by weight, preferably 0 to 100 parts by weight, more preferably 0 to 50 parts by weight.
The content of the hydroxyl group in the polyurethane resin is preferably, 3 to 20 pieces per one molecule, more preferably, 4 to 5 pieces per one molecule. If the amount less than 3 pieces per molecule, the reactions with the polyisocyanate hardener are reduced, thereby likely lowering the coating film strength and the durability. If the amount is more than 20 pieces, the solubility to the solvent and the dispersing property are likely lowered.
To adjust the content of the hydroxyl groups in the polyurethane resin, a compound having trifunctional or higher hydroxyl groups can be used. Specifically, exemplified are trimethylolethane, trimethylolpropane, trimellitic acid anhydride, glycerin, pentaerythritol, hexanetriol, dibasic acid made from polyesterpolyol as set forth in Japanese Patent Publication (KOKOKU) Heisei No. 6-64,726, branched polyester having trifunctional or higher hydroxyl groups obtained from the dibasic acid as a glycol component, and polyetherester. Trifunctional resins are preferable, and if the resin is tetrafunctional or higher, the resin is easily gelled during the reaction process.
The polyurethane resin preferably contains at least one type of polar groups selected from xe2x80x94SO3M, xe2x80x94OSO3M, xe2x80x94COOM, xe2x80x94PO3MMxe2x80x2, xe2x80x94OPO3MMxe2x80x2, xe2x80x94NRRxe2x80x2, and xe2x80x94N+RRxe2x80x2Rxe2x80x3COOxe2x88x92 (wherein each of M and Mxe2x80x2 denotes a hydrogen atom, an alkali metal, an alkaline earth metal, or ammonium salt, and R, Rxe2x80x2 and Rxe2x80x3 represents alkyl groups having a carbon number of 1 to 12, respectively), and more particularly, xe2x80x94SO3M, xe2x80x94OSO3M. The amount of those polar groups is preferably, 1xc3x9710xe2x88x925 to 2xc3x9710xe2x88x924 eq/g, more preferably, 5xc3x9710xe2x88x925 to 1xc3x9710xe2x88x924 eq/g. If the amount is less than 1xc3x9710xe2x88x925 eq/g, adhesion to the particles may become inadequate, and the dispersing property may be lowered. If the amount is more than 2xc3x9710xe2x88x924 eq/g, the resin loses solubility to the solvent, so that the dispersing property may be lowered.
The mean molecular weight number (Mn) of the polyurethane resin is preferably of 5,000 to 100,000, more preferably, 10,000 to 50,000, further preferably, 20,000 to 40,000. If it is less than 5,000, the coating film has a weak strength and low durability. If it is more than 100,000, the resin has a low solubility to the solvent and a low dispersing property.
The ring structure of the polyurethane resin affects the rigidity of the resin, and the ether group contributes to the softness of the resin. The polyurethane resin has a high solubility, a large inertia radius (dispersions of molecules), and a good dispersing property of the particles. The resin also has two features of the polyurethane resin itself, hardness (high Tg, and high Young""s modulus), and tenacity (extension).
The paints for forming magnetic layers can contain, in addition to the above components, normally used additives or fillers such as abrasives such as xcex1xe2x80x94Al2O3, Cr2O3, antistatic agents such as carbon black, lubricants such as aliphatic acid, aliphatic acid ester, silicone oil, and dispersants.
The magnetic layer of the magnetic recording medium of the invention preferably has a Tg of 30xc2x0 C. or higher but of 150xc2x0 C. or less in terms of improvements of running durability. The thickness of the magnetic layer is preferably of 0.03 to 0.5 xcexcm, more preferably 0.05 to 0.3 xcexcm in terms of sharp magnetic reversions to enhance the digital recording performance. The magnetic recording medium of the invention further has the squareness of 0.82 or higher and SFD of 0.5 or less in terms of high output and high easing property.
The magnetic recording medium of the invention widely includes a structure having a magnetic layer on one side of the aromatic polyamide film. The magnetic recording medium of the invention also includes a structure having a layer or layers other than the magnetic layer. For example, the medium can have a back coating layer provided on the side opposite to the magnetic layer, a nonmagnetic layer containing nonmagnetic powders, a soft magnetic layer containing soft magnetic powders, a second magnetic layer, a cushion layer, an over coating layer, an adhesive layer, and a protection layer. Each of those layers can be formed at an appropriate position so that function of each layer can be displayed effectively.
A desirable magnetic recording medium of the invention is a magnetic recording medium having a nonmagnetic layer containing nonmagnetic inorganic particles and a binder between the aromatic polyamide film and the magnetic layer. The nonmagnetic inorganic particles can be selected from inorganic compounds or nonmagnetic metal such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. As inorganic compounds, exemplified are, e.g., titanium oxide (TiO2, TiO), xcex1-alumina having an alpha-conversion ratio of 90% to 100%, xcex2-alumina, xcex3-alumina, xcex1-iron oxide, chrome oxide, zinc oxide, tin oxide, tungsten oxide, vanadium oxide, silicon carbide, cerium oxide, corundum, silicon nitride, titanium carbide, silicon dioxide, magnesium oxide, zirconium oxide, boron nitride, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfate, goethite, aluminum hydroxide, and so on solely or in combination. Particularly, titanium dioxide, zinc oxide, iron oxide, barium sulfide are preferable, and titanium dioxide, and iron oxide are more preferable. As a nonmagnetic metal, Cu, Ti, Zn, Al and so on are exemplified. The mean particle diameter of those nonmagnetic powders is 0.005 to 2 xcexcm, but nonmagnetic powders having a different mean particle diameter may be combined when necessary, or |nonmagnetic powders of one type having a wider profile of particle diameters may be used to obtain substantially the same result. Particularly, it is preferable that the nonmagnetic powders have the mean particle diameter of 0.01 xcexcm to 0.2 xcexcm. The pH of the nonmagnetic powders is preferably of 6 to 9. The nonmagnetic powders have a specific surface area of 1 to 100 m2/g, preferably 5 to 50 m2/g, more preferably, 7 to 40 m2/g. The nonmagnetic powders preferably have a crystallite size of 0.01 xcexcm to 2 xcexcm. The oil absorption amount using the DBP is 5 to 100 ml/100 g, preferably 10 to 80 ml/100 g, more preferably, 20 to 60 ml/100 g. The specific gravity is 1 to 12, preferably 3 to 6. The shape can be any of an acicular shape, spindle shape, spherical shape, polygonal shape, and plate shape.
Those for the magnetic layer are applicable to the binders, the lubricants, the dispersants, the additives, the solvents, and the dispersing methods and the like for the nonmagnetic layer. Particularly, publicly known technologies for the magnetic layers are also applicable to the amount and kind of the binder, and the amount and kind of the additives and dispersants.
With respect to the thickness of the layers, the magnetic layer can be, for example, 0.03 to 1 xcexcm, preferably 0.05 to 0.5 xcexcm, more preferably 0.05 to 0.3 xcexcm, whereas the nonmagnetic layer can be, for example, 0.1 to 3 xcexcm, preferably 0.5 to 3 xcexcm, more preferably 0.8 to 3 xcexcm. The thickness of the nonmagnetic layer is preferably thicker than the thickness of the magnetic layer. A magnetic recording medium may preferably have two magnetic layers. In such a case, for example, the upper layer is 0.2 to 2 xcexcm, preferably 0.2 to 1.5 xcexcm, and the lower layer is 0.8 to 3 xcexcm. Where the magnetic layer is formed solely, the thickness is ordinarily 0.1 to 5 xcexcm, preferably 0.1 to 3 xcexcm, and more preferably 0.1 to 1.5 xcexcm. Where a soft magnetic layer is formed between the aromatic polyamide film and the magnetic layer, for example, the thickness of magnetic layer can be set to 0.03 to 1 xcexcm, preferably 0.05 to 0.5 xcexcm, and that of the soft magnetic layer can be set to 0.8 to 3 xcexcm.
The thickness of the back coat layer formed on the magnetic recording medium of the invention is preferably set n a range of 0.05 to 0.5 xcexcm, more preferably in a range of 05 to 0.4 xcexcm, further preferably in a range of 0.05 to 3 xcexcm.
Particulate oxides are preferably used to the back coating layer of the magnetic recording medium of the invention. As such particulate oxides, any of titanium ides, xcex1-iron oxides, and the mixture of those can be used. Ordinary titanium oxides and xcex1-iron oxides can be used here.
The shape of the particles is not specifically limited. In a case of a spherical shape, proper particles have a particle size of 0.01 to 0.1 xcexcm, and in a case of an acicular shape, it is proper to have an acicular aspect of 2 to 20 and a preferable major axis length of 0.05 to 0.3 xcexcm.
A part of the surface of the particulate oxides can be modified to other compounds or can be covered wit other compounds such as, e.g., Al2O3, SiO2, and ZrO2.
The back coating layer may preferably contain carbon black to prevent static from building up. As the carbon black used for the back coating layer, those ordinarily used for magnetic recording tapes can be used widely. For example, used are furnace black for rubbers, thermal black for rubbers, coloring carbon black, acetylene black, and so on. The carbon black preferably has a particle diameter of 0.3 xcexcm or smaller to prevent undulations on the back coating layer from transferring to the magnetic layer. The carbon black more preferably has a particle diameter of 0.01 to 0.1 xcexcm. The used amount of the carbon black in the back coating layer is preferably set 1.2 or less as the optical transparent concentration (transmission amount measured by TR-927 (product name)) made by Macbeth Co.
To improve the running durability, it is advantageous to use carbon blacks of two types having different mean particle sizes. In such a case, a desirable combination is made of a first carbon black having the mean particle size in a range of 0.01 to 0.04 xcexcm and a second carbon black having the mean particle size in a range of 0.05 to 0.3 xcexcm. The desirable content of the second carbon black is 0.1 to 10 parts by weight where the total amount of the particle oxides and the first carbon black is 100 parts by weight, and more preferably 0.3 to 3 parts by weight.
The weight ratio of the particulate oxides to the carbon black is set to 60/40 to 90/10, more preferably, 70/30 to 80/20. Where the particulate oxides are contained more than the carbon black, a back coating layer can be formed with a good dispersing property of the particles and a smooth surface. The paint for forming the back coating layer having such compositions has a high thixotrophy in comparison with conventional paints for forming back coating layer essentially containing carbon blacks. Therefore, the paint can be coated by an extrusion method or a gravure method because of its high concentration. By application of such a high concentration paint, a back coating layer can be formed which strongly adhering to the support even through having a thin film thickness and which having a high dynamic strength.
The used amount of the binder is selected from a range of 10 to 40 parts by weight where the total amount of the particulate oxides and the carbon black is 100 parts by weight, and more preferably, 20 to 32 parts by weight. The film strength of the back coating layer thus formed is high and has a low surface electric resistance.
For the binder for the back coating layer in the invention, conventionally known thermoplastic resins, thermosetting resins, reaction type resins can be used. The dried thickness of the back coating layer is about, ordinarily, 0.2 to 1 xcexcm, and more preferably, 0.2 to 0.6 xcexcm.
The magnetic recording medium of the invention can have a tape thickness of 4 to 9 xcexcm because the back coating layer is hardly transferred onto the magnetic layer even where the layer is wound up and kept with a high tension.
The magnetic recording medium of the invention can be manufactured by applying a paint or evaporating a material on the surface of the running aromatic polyamide film so that the layer thickness after dried comes in, e.g., the above prescribed range. Plural magnetic paints or nonmagnetic paints can be applied sequentially or simultaneously as multilayered. As a coater for such magnetic paints, coaters for air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeezing coating, dipping coating, reverse roller coating, transfer roller coating, gravure coating, kiss coating, cast coating, spray coating, spin coating, and so on can be used. Those are referred in, for example, xe2x80x9cSaisin Coating Gijyutu (Coating Technology Updated)xe2x80x9d published by Kabushiki Kaisha Sogo Gijyutu Center (Showa 58 (1983) May 31).
When a magnetic recording tape having two or more layers on one side is manufactured, the following methods can be used.
1. A lower layer is first applied with a coating apparatus commonly used for application of magnetic paints, e.g., a gravure coating, roll coating, blade coating, or extrusion coating apparatus, and an upper layer is then applied, while the lower layer is in a wet state, by means of a support-pressing extrusion coater such as those disclosed in U.S. Pat. Nos. 4,480,583; 4,681,062; and 5,302,206.
2. An upper layer and a lower layer are applied almost simultaneously using a single coating head having therein two slits for passing coating fluids, such as those disclosed in U.S. Pat. Nos. 4,854,262; 5,072,688; and 5,302,206.
3. An upper layer and a lower layer are applied almost simultaneously with an extrusion coater equipped with a back-up roll, such as that disclosed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-174965.
The back coating layer can be prepared by applying a paint for forming the back coating layer in which particulate components such as abrasives and antistatics and the binder are dispersed in an organic solvent on a side opposite to the side of the magnetic layer. If the particulate oxides are used more than the carbon black as in the preferred example as described above, the paint for forming the back coating layer can be prepared without roll kneading, which is used to be required conventionally, because an adequate dispersion can be guaranteed. If a carbon black containing ratio is low, the remaining cyclohexane amount can be reduced after the drying process even where a cyclohexane is used as a solvent.
The coated magnetic layer is dried after the ferromagnetic field powders contained in the magnetic layer are subject to a processing for magnetic field orientation. The processing for magnetic field orientation can be done by a method widely known to persons skilled in the art.
The magnetic layer is processed to have the surface of the magnetic layer smoother using a super calendar roller or the like after dried. By such a surface smoothing process, voids created by removals of the solvent when dried are eliminated, and the filling rate of the ferromagnetic powders is improved. Therefore, a magnetic recording tape can be produced with a high electromagnetic characteristics.
As a calendar processing roller, a heat resistance plastic roller made of epoxy, polyimide, polyamide, polyamideimide, and so on can be used. A metal roller may be used for the processing.
The magnetic recording medium according to the invention preferably has a well-smooth surface. To render the surface smooth, it is effective to use, e.g., the calendar processing over the magnetic layer formed upon selecting a specific binder as described above. The calendar processing is implemented by setting the temperature of the calendar at 60 to 100xc2x0 C., preferably, 70 to 100xc2x0 C., more preferably 80 to 100xc2x0 C., the pressure at 100 to 500 kg/cm, preferably, 200 to 450 kg/cm, further preferably 300 to 400 kg/cm. The obtained magnetic recording tape can be used upon slitting it into a prescribed size using a cutting machine or the like. The magnetic recording tape subjecting to the calendar processing is generally thermally treated.
The magnetic recording medium of the invention preferably has a center-face surface roughness Ra, measured by a light interference type surface roughness meter, of 5.0 nm or less, preferably 4.5 nm or less where the measured range is 121 xcexcmxc3x9792 xcexcm, and of 8.5 nm or more and 21.5 nm or less where the measured range is 1.2 mmxc3x970.9 mm. Where the medium has such undulations, the magnetic recording medium advantageously has good electromagnetic characteristics and good running durability.